I used to be pretty anti-vegetarian because I'd had a lot of run-ins with vegans that were very annoying and pretentious, but I've calmed down a lot. I've even had a few thoughts about trying the lifestyle just to see how it would work out for me now that I mostly buy my own food. I doubt it would stick, but who knows. My reasons would probably be more health- and self-discipline-related than anything, though.
As for not eating meat for environmental reasons, I know just one person going vegetarian wouldn't make much of a difference, but still a person may know they're not changing anything by themselves, but still feel like they don't want to contribute to the massive effects that huge amounts of livestock have on the environment. And, like Watershed said, a larger number of vegetarians would mean that demand for livestock would drop, therefore reducing their numbers and allowing them to be raised more humanely and have less impact on the environment.
As for not eating meat for environmental reasons, I know just one person going vegetarian wouldn't make much of a difference, but still a person may know they're not changing anything by themselves, but still feel like they don't want to contribute to the massive effects that huge amounts of livestock have on the environment. And, like Watershed said, a larger number of vegetarians would mean that demand for livestock would drop, therefore reducing their numbers and allowing them to be raised more humanely and have less impact on the environment.